Polyolefins are widely used commercially because of their robust physical properties. For example, various types of polyethylenes, including high density, low density, and linear low density polyethylenes, are some of the most commercially useful. Polyolefins are typically prepared with a catalyst that polymerizes olefin monomers. Therefore, there is interest in finding new catalysts and catalyst systems that provide polymers having improved properties.
Low density polyethylene is generally prepared at high pressure using free radical initiators, or in gas phase processes using Ziegler-Natta or vanadium catalysts. Low density polyethylene typically has a density in the range of 0.916 to 0.950 g/cm3. Typical low density polyethylene produced using free radical initiators is known in the industry as “LDPE.” LDPE is also known as “branched” or “heterogeneously branched” polyethylene because of the relatively large number of long chain branches extending from the main polymer backbone. Polyethylene in the same density range, e.g., 0.916 to 0.940 g/cm3, which is linear and does not contain long chain branching, is known as “linear low density polyethylene” (“LLDPE”) and is typically produced by conventional Ziegler-Natta catalysts or with metallocene catalysts. “Linear” means that the polyethylene has few, if any, long chain branches, typically referred to as a g′vis value of 0.97 or above, such as 0.98 or above. Polyethylenes having still greater density are the high density polyethylenes (“HDPEs”), e.g., polyethylenes having densities greater than 0.940 g/cm3, and are generally prepared with Ziegler-Natta catalysts or chrome catalysts. Very low density polyethylenes (“VLDPEs”) can be produced by a number of different processes yielding polyethylenes having a density less than 0.916 g/cm3, typically 0.890 to 0.915 g/cm3 or 0.900 to 0.915 g/cm3.
Polyolefins, such as polyethylene, which have high molecular weight, generally have desirable mechanical properties as compared to their lower molecular weight counterparts. However, high molecular weight polyolefins can be difficult to process and can be costly to produce. As used herein, “high molecular weight” is defined as a number average molecular weight (Mn) value of 100,000 g/mol or more. “Low molecular weight” is defined as an Mn value of less than 100,000 g/mol.
Nonetheless, polyolefin compositions formed by catalysts capable of forming high molecular weight polyolefins typically also have a broad molecular weight distribution, as indicated by high polydispersity indices, and/or the polyolefins are of such high molecular weight (e.g., Mw of 1,500,000 g/mol) as to have processing difficulties due to hardness. Furthermore, catalysts capable of forming high molecular weight polyolefins typically have low polymer productivity.
There is a need for catalysts capable of forming polyolefins, such as polyethylene, with high molecular weight and narrow molecular weight distribution capable of high polyolefin productivity.
References of interest include: US 2015/0322184.